Spray booths and other work areas in which solvents or other hazardous materials are used typically employ a fully or partially enclosed, climate controlled space in which work pieces are coated, stripped, or otherwise processed. Forced draft ventilation air is generally driven through these enclosures to achieve a variety of objectives, including a) removal of overspray particulate, solvent vapors, and/or other hazardous constituents from the worker stationed in or near the area; b) maintaining constituent concentrations in the vicinity of workers present in the enclosure below established safety levels; c) maintaining solvent concentrations in the enclosure and exhaust ducts below lower explosive limit (LEL) concentrations; and/or d) maintaining a clean environment to avoid corrupting the workpiece surface with unwanted contaminants. The minimum required volumetric flow rate through an enclosure is dictated by the enclosure configuration as well as applicable regulations; typically a linear flow rate of 100 feet per minute or more is employed for paint spray operations.
The ventilation air exiting the enclosure often passes through a particulate filtration system before it is exhausted to atmosphere. Occasionally, the exhaust air is vented to an air pollution control system to remove the volatile organic compounds and/or other hazardous constituents before it is vented to atmosphere. This "once-through" mode of operation typically employed for enclosure ventilation systems results in rather high facility air handling costs (e.g. heating, cooling), as well as high air pollution emission control costs due to the large exhaust volume flow rates.
One means that has been used in the field to reduce exhaust air flow rate is the recirculation of a portion of the air back into the enclosure; the remainder is vented either to atmosphere or to an air pollution emission control device. Recirculated air is mixed with fresh make-up air that is brought in to replace the exhaust air. Recirculation enhances the overall cost effectiveness of facility operation, because it reduces air conditioning costs (e.g. heating, cooling) as well as air pollution control system installation and operating costs.
Though there are several ventilation systems known in the art that employ recirculation, every one of them relies on a fixed rate of recirculation, and therefore a fixed recirculated volume flow rate and a fixed exhaust volume flow rate. One reason that the recirculation volume flow rate is fixed is in consideration of applicable safety rules, which stipulate that constituent concentrations in the recirculated stream not exceed a safe level. Thus, recirculation system flow rates are designed and fixed such that constituent concentrations remain at safe levels, even under "worst-case" (i.e. high solvent usage) conditions.
There are no known dynamically optimized recirculating ventilation systems in the prior art. Therefore, it remains an unsolved problem in field to both maintain safe levels of constituent concentrations in the recirculation stream, and continually maximize the recirculation rate to achieve minimal exhaust volume flow rates and thus the costs associated therewith.
In another patent by this inventor, U.S. Pat. No. 5,221,230, a paint spray booth is disclosed in which a split-flow plate is used to segregate a portion of the exhaust air, which is vented to an air pollution control device. The remainder is either recirculated or vented to atmosphere. The location of the split-height is fixed; thus the volume flow rate in each of the two streams created by the "split-flow" plate does not vary. Moreover, the split-height location must be calculated by skilled engineering professionals in order to correctly implement this invention. There is also an inherent danger of the split-height being miscalculated, which could lead to injury. Furthermore, it is based upon a limiting condition that the air flow be not only laminar, but either vertical or horizontal in relation to the walls thereof. Finally, this invention requires that a constituent stratification pattern which occurs naturally in the booth be preserved throughout the booth.
Another paint spraying assembly that employs recirculation to reduce the exhaust flow rate is disclosed by U.S. Pat. No. 4,266,504, by Roesher. This assembly employs a safety device to monitor the hazardous constituent concentrations in the booth; the monitor also causes air to be exhausted through an exhaust duct rather than the recirculation duct if the concentration exceeds a safe level. However, this patent does not teach the application of a monitor to continually optimize the ventilation system, or dynamically vary the recirculation volume flow, and the advantages thereof. The sole purpose of the monitor is to maintain the enclosure concentration below 25% of the LEL; no monitoring of the recirculation stream is even included. This invention employs a fan and blower system to move the recirculated air at a constant rate, thus the recirculation volume flow rate is constant and unchanging. U.S. Pat. No. 4,515,072 by Crisp also discloses a spray booth that distributes intake ventilation air through the top of the booth. During painting, the booth operates in a "once-through" mode, where 100% of the air is exhausted to atmosphere. After painting, the system switches to recirculation mode, wherein a fixed (95%) percent of ventilation air is recirculated into the booth, and a fixed (5%) of the air is passed to a washer and then to atmosphere. In order to function, this invention relies upon their being laminar flow in the intake ducts (to create the necessary velocity/pressure changes therein) and on a downward air flow from the top of the enclosure. The volumetric flow rates associated with the recirculation and "once-through" modes are fixed; the mode of operation depends only on painting activity in the booth. A monitor is not included in the description, and is therefore not required for ventilation system operation.
In U.S. Pat. No. 5,276,566, techniques for fluctuating the venting of fluid flow from a chamber are identified to control eddy formation around an object in an enclosure. Fluctuation of the direction of fluid flow by varying inlet cross sections or by sliding members across an inlet flow is disclosed. Another embodiment allows the venting of portions of a flow by means of a flapping valve. But the method, structure, and purpose of the fluctuating flow is completely different than in the present invention.
This is the first invention wherein, through variable flows and other features, recirculation is instantaneously maximized to reduce the cost of ventilation air heating and cooling, and reduce the cost of controlling pollutant emissions. These features are needed in the field, as costs of air pollution control and heating/cooling are exceedingly high. This invention is an improvement over prior known systems of this kind.